16.5Phantom Byte Strings

A phantom byte string is a small Racket value that is
treated by the Racket memory manager as having an arbitrary size,
which is specified when the phantom byte string is created or
when it is changed via set-phantom-bytes!.

A phantom byte string acts as a hint to Racket’s memory
manager that memory is allocated within the process but through a
separate allocator, such as through a foreign library that is accessed
via ffi/unsafe. This hint is used to trigger
garbage collections or to compute the result of
current-memory-use. Currently, the hint is used only in
Racket 3m (the main variant of Racket).